The invention relates to a low-pressure mercury vapor discharge lamp comprising a discharge vessel, which discharge vessel encloses a discharge space containing a filling of mercury and an inert gas in a gastight manner, and said discharge vessel (10) comprising tubular end portions, which each have a longitudinal axis, electrodes being arranged in the discharge space for generating and maintaining a discharge in the discharge space, and at least an auxiliary amalgam being provided on a carrier in the discharge vessel in the proximity of at least one of the electrodes.
In mercury vapor discharge lamps, mercury constitutes the primary component for (efficiently) generating ultraviolet (UV) light. An inner surface of the discharge vessel may be coated with a luminescent layer comprising a luminescent material (for example a fluorescent powder) for the conversion of UV to other wavelengths, for example to UV-B and UV-A for tanning purposes (sunbed lamps) or to visible radiation for general lighting purposes. As a result, such discharge lamps are also referred to as fluorescent lamps. The discharge vessel of low pressure mercury vapor discharge lamps generally is tubular in shape with a circular cross-section and comprises both elongated and compact embodiments. In general, the tubular discharge vessel of so-called compact fluorescent lamps comprises a collection of relatively short straight parts having a relatively small diameter, which straight parts are connected to each other, on the one hand, by means of bridge parts and/or, on the other hand, by means of curved parts. Compact fluorescent lamps are generally provided with an (integrated) lamp cap.
In the description and the claims of the current invention, the designation xe2x80x9cnominal operationxe2x80x9d is used to indicate operating conditions where the mercury vapor pressure is such that the radiant efficacy of the lamp is at least 80% of that during optimum operation, i.e. operating conditions where the mercury vapor pressure is optimal. Furthermore, in the description and the claims, the xe2x80x9cinitial radiant efficacyxe2x80x9d is defined as the radiant efficacy of the discharge lamp 1 second after switching on the discharge lamp, and the xe2x80x9crun-up timexe2x80x9d is defined as the time required by the discharge lamp to reach a radiant efficacy of 80% of that during optimum operation.
A low-pressure mercury vapor discharge lamp as mentioned in the opening paragraph is known from U.S. Pat. No. 5,204,584. Said known low-pressure mercury vapor discharge lamp comprises a suitable combination of a main amalgam and an auxiliary amalgam, the latter being provided on one of the current supply conductors which extend from the electrodes through a so-called stem in the tubular end portion and, subsequently, issue from the discharge vessel to the exterior. In this known lamp the main amalgam is dosed so that it controls the mercury vapor pressure discharge space except for the starting period. FIG. 5 shows this known lamp, having a main amalgam 131 and auxiliary amalgams 133 and 135.
In general, a low-pressure mercury vapor discharge lamp containing an auxiliary amalgam with sufficient mercury has a relatively short run-up time. Upon switching on the lamp, the auxiliary amalgam is heated by the electrode, so that the auxiliary amalgam relatively rapidly delivers a substantial part of the mercury it contains. It is desirable that, prior to switching on the lamp, said lamp has been out of operation for a sufficiently long time to enable the auxiliary amalgam to absorb sufficient mercury. If the lamp has been out of operation for a relatively short period of time, the run-up time reduction is only small and, in addition, the initial radiant efficacy is (even) lower than that of a lamp comprising only a main amalgam because the auxiliary amalgam sets the mercury vapor pressure in the discharge space at a relatively lower value. In addition, relatively long lamps exhibit the drawback that relatively much time goes by before the mercury delivered by the auxiliary amalgam has spread throughout the discharge vessel, so that after switching on such lamps there is a relatively bright zone near the auxiliary amalgam and a relatively dark zone at some distance from the auxiliary amalgam, which zones last a few minutes.
A drawback of the known low-pressure mercury vapor discharge lamp resides in that the run-up time is relatively long in spite of the application of an auxiliary amalgam.
It is an object of the invention to provide a lamp of the type described in the opening paragraph which, when regularly used, has a relatively short run-up time. To achieve this, the low-pressure mercury vapor discharge lamp is characterized in accordance with the invention in that a part of the carrier is arranged in a plane transverse to the longitudinal axis.
As a result of the fact that said part of the carrier is arranged in a plane transverse to the longitudinal axis, the auxiliary amalgam is better irradiated by the heat generated in the electrode when the low-pressure mercury vapor discharge lamp is started. As a result, relatively more mercury is loosened relatively more rapidly from the auxiliary amalgam upon starting the discharge lamp. By virtue of the measure in accordance with the invention, the run-up time of the low-pressure mercury vapor discharge lamp is relatively short. In low-pressure mercury vapor discharge lamps, the electrode is generally arranged transversely to the longitudinal axis.
In the known discharge lamp, the auxiliary amalgam is provided on one of the current supply conductors, which extend from the electrodes through a so-called stem in the tubular end portion and, subsequently, issue from the discharge vessel to the exterior. As a result, the auxiliary amalgam is arranged so as to be parallel to the longitudinal axis, thus rendering the auxiliary amalgam relatively insensitive to radiant heat emitted by the electrodes upon starting the low-pressure mercury vapor discharge lamp.
Relative to the known discharge lamp, the auxiliary amalgam in the low-pressure mercury vapor discharge lamp in accordance with the invention is provided so as to be rotated through 90xc2x0 as it were, so that the major part of the carrier is positioned so as to extend parallel to the electrode. Without being obliged to give any theoretical explanation, the inventors hold the opinion that the reduction of the run-up time is caused by the fact that a part of the mercury, which is loosened from the auxiliary amalgam on the carrier, travels more toward the interior of the discharge vessel of the low-pressure mercury vapor discharge lamp. As a result, in a short period of time after starting the discharge lamp, the back diffusion of mercury to the cold locations at the side of the electrodes facing away from the discharge space takes longer than in the known lamp. By virtue thereof, more mercury is available at locations in the discharge space which heat up relatively rapidly, thereby giving rise to an improved run-up behavior.
An embodiment of the low-pressure mercury vapor discharge lamp is characterized in accordance with the invention in that a stem in the tubular end portion carries the electrode, and in that the stem comprises a supporting body on which the carrier is provided. As will be explained in greater detail hereinbelow, the supporting body is formed by an extended exhaust tube or by a supporting wire provided in the stem. In an alternative modification, the carrier is provided directly on the stem. In the known discharge lamp, the auxiliary amalgam is provided on one of the current supply conductors, which extend from the electrodes through a so-called stem in the tubular end portion and, subsequently, issue from the discharge vessel to the exterior. Particularly in so-called cold-start low-pressure mercury vapor discharge lamps, this causes material to be sputtered off the auxiliary amalgam and deposited on the tubular end portions of the discharge vessel. Without being obliged to give any theoretical explanation, the inventors hold the opinion that the above problem is caused by the cold ignition of the discharge lamp, whereby, shortly after starting the lamp, the discharge acts on the (still) cold emitter and on the auxiliary amalgam, resulting in a discharge on the emitter and on the auxiliary amalgam. The discharge on the auxiliary amalgam causes the amalgam to be sputtered off the carrier and gives rise to blackening of the tubular end portions and a poor run-up behavior. Sputtering as well as blackening are undesirable phenomena. Furthermore, the service life of the lamp is reduced if any amalgam originating from the auxiliary amalgam lands on the electrode. The effect is enhanced by the relatively high cathode drop which is characteristic of cold-ignition low-pressure mercury vapor discharge lamps.
In a preferred embodiment of the low-pressure mercury vapor discharge lamp in accordance with the invention, the carrier is arranged at a side of the electrode facing away from the discharge space. In this embodiment, the carrier is situated between the stem and the electrode. By virtue thereof, a simple construction is obtained having a relatively short supporting wire supporting the carrier.
In an alternative, favorable embodiment of the low-pressure mercury vapor discharge lamp in accordance with the invention, the carrier is electrically insulated with respect to the electrode. As the carrier with the auxiliary amalgam is electrically insulated from the current supply conductor connected to the electrode, said auxiliary amalgam is at the same potential as its surroundings and will not attract ions present in the discharge. As a result, the run-up behavior of the discharge lamp is improved and blackening of the tubular end portions of the low-pressure mercury vapor discharge lamp is substantially precluded. The auxiliary amalgam on the carrier is heated by the discharge in a manner comparable to that in induction lamps.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.